Fertilizers have been used for some time to provide nitrogen to the soil. The most widely used and agriculturally important nitrogen fertilizer is urea, CO(NH2)2. Most of the urea currently produced is used as a fertilizer in its granular (or prilled) form. After application of urea to soil, it is readily hydrolyzed to yield ammonia and carbon dioxide. This process is catalyzed by the enzyme urease, which is produced by some bacteria and fungi that may be present in the soil. The gaseous products formed by the hydrolysis reaction (i.e., ammonia and carbon dioxide) can volatilize to the atmosphere and thus, substantial losses from the total amount of the nitrogen applied to the soil can occur.
Attempts to reduce losses of applied nitrogen have utilized urease inhibitors and/or nitrification inhibitors as additives to the fertilizer. Urease inhibitors are compounds capable of inhibiting the catalytic activity of the urease enzyme on urea in the soil. Nitrification inhibitors are compounds capable of inhibiting the bacterial oxidation of ammonium to nitrate in the soil. Urease inhibitors and nitrification inhibitors can be associated with fertilizers in various ways. For example, they can be coated onto fertilizer granules or mixed into fertilizer matrices. A number of granulation methods are known, including falling curtain, spherudization-agglomeration drum granulation, prilling and fluid bed granulation technologies.
Examples of urease inhibitors are the thiophosphoric triamide compounds disclosed in U.S. Pat. No. 4,530,714 to Kolc et al., which is incorporated herein by reference. The disclosed thiophosphoric triamide compounds include N-(n-butyl) thiophosphoric triamide (NBPT), the most developed representative of this class of compounds. When incorporated into a urea-containing fertilizer, NBPT reduces the rate at which urea is hydrolyzed in the soil to ammonia. The benefits realized as a result of the delayed urea hydrolysis include the following: (1) nutrient nitrogen is available to the plant over a longer period of time; (2) excessive build-up of ammonia in the soil following the application of the urea-containing fertilizer is avoided; (3) the potential for nitrogen loss through ammonia volatilization is reduced; (4) the potential for damage by high levels of ammonia to seedlings and young plants is reduced; (5) plant uptake of nitrogen is increased; and (6) an increase in crop yields is attained. NBPT is commercially available for use in agriculture and is marketed in such products as the AGROTAIN® nitrogen stabilizer product line.
Industrial grade NBPT is a solid, waxy compound, and decomposes by the action of water, acid and/or elevated temperature. In particular, NBPT is believed to degrade at elevated temperatures into compounds that may not provide the desired inhibitory effects on the urease enzyme. Accordingly, its combination with other solid materials to provide a material capable of inhibiting urease, particularly via granulation with urea (which generally employs heat) can be challenging. As such, there is a need for urease inhibitor-containing compositions that can be combined with urea, desirably using current urea manufacturing practices, so as to produce fertilizer compositions that provide effective urease inhibition.